Firmware lock unintentionally activated and Apple is unable to help me unlock my Mac Mini.

I bought a used 2014 Mac Mini from an authorized 3rd party dealer. Somehow the system lock was activated and I was asked for a 6 digit pin. Tried anything I would used for such a pin and it then locked me out completely after a few tries. Now it says "Wrong passcode. Try again after 60 minutes." But after 60 minutes I am not given any means to enter a passcode. I tried booting it in disk recovery mode and I get a black screen with a white padlock. Under is a box where I can enter something but nothing works. I took it to the store where I bought it and they said that the firmware lock had to have been set after I bought the machine because they would not have been able to refurbish the machine and install the new OS if that lock was set. I suspect the lock was set by the original owner but they say not possible and sent me to the local Apple certified dealer (not a Apple store) with my proof of purchase. That store said that Apple rejected my proof of purchase and that I needed the original proof of purchase. So before I go back and argue with the store where I bought it, I need to know

1) How is this lock activated? Could I have someone activated it when I upgraded to Catalina? (I think I did that upgrade, however I can see this machine in my iCloud account and it says that it has Mojave, so I am not sure as this is one of several machines I manage for my non-profit.)

2) Is the claim of the first store correct that they could not have refurbished the drive if it had the system lock on it?